1. Field of the Invention
This invention relates to a colorless transfer-onto-plain paper type pressure-sensitive copying paper.
2. Description of the Prior Art
A heretofore known pressure-sensitive copying paper is constituted of a "top sheet" and a "bottom sheet" where the top sheet is obtained by coating to a reverse surface of a substrate microcapsules containing a color former-containing oil as a core material prepared by dissolving a colorless electron-donating color former in a non-volatile oil and the like, and the bottom sheet is obtained by coating a colorless electron-accepting color developer to a right surface of another substrate. When two sheets are superposed so that their coated surfaces confront each other, and a pressure is applied thereto with a typewriter or the like, a colored image is obtained on the bottom sheet.
In obtaining three or more copied sheets, the required number of intermediate sheets produced by coating a color developer and microcapsules containing a color former to the right and reverse surfaces of a substrate, respectively, are inserted between the top and bottom sheets. There is also the so-called self-contained type pressure-sensitive copying paper produced by coating both microcapsules containing a color former and a color developer to the same surface of a substrate. Since no coloring material is used in such copying paper, it does not soil hands or clothes, and is widely used as office paper and the like. However, such copying paper has the defects that when it is used, at least two, usually three types of coated papers, i.e., top sheets, intermediate sheet and bottom sheet should be produced, and that a copy image can be obtained only on the intermediate sheet, the bottom sheet or a substrate coated with a developer or the like of self-contained type pressure-sensitive paper.
It has already been known that in the above-mentioned self-contained type pressure-sensitive copying paper, when the coated surface is placed on a plain paper and a pressure is applied thereto, a colored image is obtained only on the coated surface and no copy image is obtained on the plain paper.
The plain paper used herein means a substrate having a transfer side on which neither electron-donating color former nor electron-accepting color developer is present.
Pressure-sensitive copying papers in which both color former and color developer are present on the same surface of a substrate and which can form a copy image on a plain paper are disclosd in Japanese Patent Kokai (Laid-Open) No. 126111/1979, Japanese Patent Kokoku (Post-Exam Publn) No. 16728/1978 and Japanese Patent Kokai (Laid-Open) No. 159008/1979.
In Japanese Patent Kokai (Laid-Open) No. 126111/1979, it is disclosed that a coating solution prepared by adding a solid color former and a solid color developer to wax is single coated onto a substrate, and it is clearly stated that immediately after the color former and the color developer are mixed, said coating solution undergoes coloration reaction and is colored. That is to say, said Japanese Patent Kokai relates to a pressure-sensitive transfer material having a colored coating layer, such as the so-called backed carbon paper, and such a material is disadvantageous in appearance because the coating layer is colored. Japanese Patent Kokoku (Post-Exam Publn) No. 16728/1978 relates to chelate coloration, and according to it, a substrate is coated with a encapsulated reactant and then a solution of a coreactant in a solvent, and since the solvent is used, a material for the wall of the microcapsules should be resistant to the solvent and hence is limited. Such copying paper has many environmental and economic disadvantages.
In Japanese Patent Kokai (Laid-Open) No. 159008/1979, there is disclosed the use of waxes and a mode of the so-called bilayer coating in which a substrate has been coated with microcapsules which enclose oil containing a color former together with a binder, and then with a layer containing wax and a color developer. According to this mode, the substrate should be coated with a color developer after coating its whole surface with a microcapsule layer by means of an air knife coating machine or the like. Therefore an additional production process is required so that the cost increases. The microcapsules are applied together with a binder to a substrate in the same manner as in the case of the top sheet of commercially available pressure-sensitive copying paper, and therefore the microcapsule layer is hardly transferred onto plain paper, and on writing, the capsules are broken and a part of the liberated oil containing a color former moves into the substrate so that the amount of the color former participating in coloring is limited, and this is disadvantageous from the viewpoint of transferability and coloring property.
In Japanese Patent Kokoku (Post-Exam Publication) No. 12255/1973, a process for producing a cold-set ink obtained by mixing microcapsules with wax is disclosed. According to said patent publication, the cold-set ink prepared by mixing microcapsules containing a color former with wax is applied to the reverse side of paper to obtain the top sheet, and in order to obtain a trasferred image, the top sheet should be used in combination with the bottom sheet coated with a color developer, therefore such copying paper is disadvantageous from the viewpoint of production and use.